This invention generally relates to an electrochemical cell having an iodate in the cathode. More particularly, this invention is concerned with an alkaline electrochemical cell having an iodate and a catalyst in the cathode.
Alkaline electrochemical cells are commercially available in several standard sizes such as LR03, LR6, LR14 and LR20 which are also referred to as AAA, AA, C and D size batteries, respectively. The cells have a cylindrical shape that must comply with dimensional standards that are set by organizations such as The International Electrotechnical Commission. The cells are used by consumers to power a range of products such as cameras, compact disc players, clocks, etc. A typical cell construction includes a cylindrical container that houses an anode, a cathode, electrolyte and a separator which is positioned between the anode and the cathode.
Despite the volumetric limitations imposed by the dimensional standards, battery manufacturers constantly strive to increase the length of time that a cell, also known herein as a battery, will power a device. The cathode is one of the battery's key components that must be improved in order to provide a longer running battery. Most commercially available cylindrical alkaline batteries utilize electrolytic manganese dioxide (EMD) in the cathode as the electrochemically active material. Unfortunately, the volumetric capacity of EMD, based on a one electron reaction, is only 1540 mAh/cc which is much lower than the volumetric capacity of zinc which is 5800 mAh/cc. In order to increase the cell's run time, the useable electrochemical capacity of the cathode must be increased. One way to increase the electrochemical capacity of the cathode is to replace the EMD with another active cathode material with a volumetric capacity substantially greater than 1540 mAh/cc. For example, replacing the EMD with an iodate, such as lead iodate which has a volumetric capacity of 3,753 mAh/cc, would accomplish the objective of increasing the cathode's volumetric capacity. However, when zinc and lead iodate are coupled within a cell as opposing electrochemically active materials and the cell is discharged on a light load, such as 5 mA/g of the cathode's electrochemically active material, the cell's average running closed circuit voltage (CCV) is typically between 0.6 V and 0.8 V which is significantly lower than the thermodynamic voltage. The low CCV is believed to be due to sluggish reaction kinetics. Unfortunately, many devices that are powered by cylindrical alkaline cells cannot operate if the cell's voltage is below 0.9 V. The voltage below which a device cannot operate is commonly known as the device's functional endpoint. Many devices, such as digital still cameras and minidisk players, have functional endpoints of 1.0 V or higher. Therefore, an iodate cannot be directly substituted for EMD in cylindrical alkaline cells having an anode containing zinc because the cell's CCV is too low.
Previous attempts to utilize a cathode containing an iodate in an alkaline cell include U.S. Pat. No. 6,730,436 which discloses an alkaline cell having an anode comprising zinc and a cathode comprising copper iodate. However, in the subject patent's TABLE 1, cells made with a cathode containing copper iodate and (1) expanded graphite or (2) graphitic carbon nanofibers or (3) expanded graphite and sulfur, had average running voltages of 0.5 V, 0.6 V and 0.9 V, respectively. This patent does not teach how to manufacture an alkaline cell with a cathode comprising an iodate and an anode comprising zinc, wherein the cell, when discharged, has an average running voltage equal to or greater than 1.0 V which is the minimum closed circuit voltage needed to power many commercially available devices.
Therefore, there exists a need for an alkaline electrochemical cell that utilizes a cathode comprising an iodate, an anode comprising zinc and the cell can be discharged at 1.0 V or higher.